Heating, ventilating and air conditioning (HVAC) systems serving multiple zones are classified according to the methods used to control the temperature and humidity in each zone. An early HVAC system commonly known as terminal reheat zone control was known to have superior space temperature and humidity control capability, but it was also known for consuming large amounts of energy in the cooling mode caused by the reheating process.
Another development in zone control came in the form of variable air volume (VAV) systems that regulate the supply air volume to a space in response to temperature requirement. These VAV systems, although good at controlling temperature, cannot ensure that the proper mixture of ventilation air reaches a room or zone because the total air volume varies with temperature needs. It is therefore not uncommon for the supply air volume to be low at a time of high occupancy, when a greater need for ventilation exists.
FIG. 2 is a prior art airflow diagram of a system such as described generally, for example, in U.S. Pat. No. 6,789,618 B2 of Pearson with a face damper to regulate the volume of air passing through each heat exchanger in the system for maintaining zone temperature and a bypass damper to route excess return air to the return duct. Return air to heat exchanger is either free-flow or directly connected. Return air leaving heat exchanger and face damper is directly connected to return air duct.